The present invention relates to the field of feeding ion thrusters, and more particularly to a method and a device for feeding propellant gas to an ion thruster.
The term ion thruster is used to mean any reaction thruster, in particular for space applications, that is based on accelerating charged particles by means of an electrostatic field. This thus includes thrusters in which the particles are charged electrically by contact as well as so-called plasma thrusters, in which a plasma is generated that contains the charged particles. The invention is particularly but not exclusively applicable to feeding plasma thrusters and in particular so-called “Hall effect” thrusters having an annular channel, an anode, a magnetic circuit suitable for creating a magnetic circuit suitable for generating a magnetic field at a downstream end of the annular channel, and a cathode situated outside the downstream end of the annular channel, and in which a propellant gas, such as xenon, for example, is injected into the annular channel.
Typically, when such a Hall effect thruster is in operation, a propellant gas is injected in the proximity of the anode into the end of the annular channel. Electrons emitted by the cathode and attracted towards the anode at the end of the annular channel are trapped by the magnetic field in spiral trajectories between the two walls, thus forming a virtual cathode grid. Electrons escaping from this magnetic enclosure towards the anode come into collision with atoms of propellant gas injected into the end of the annular channel, thereby creating an ionized plasma.
The positive ions of the plasma are accelerated by the electric field that exists between the anode and the virtual cathode grid formed by the cloud of electrons trapped by the magnetic field at the open end of the annular channel. Since the mass of these positive ions is much greater than the mass of an electron, their trajectories are hardly affected by the magnetic field. The ions of this plasma jet are finally neutralized downstream from the magnetic field by electrons emitted by the cathode or produced by ionizing the plasma.
Ion thrusters have begun to be used in attitude and orbit control systems (AOCSs) for space vehicles, and in position in the AOCSs of geostationary satellites. Ion thrusters make it possible to obtain a specific impulse (Isp) that is very high, being of the order of 1500 seconds (s) for Hall effect plasma thrusters, thus making it possible to obtain accurate control over the attitude and/or the position of the vehicle while involving mass and complexity that are considerably less than would need to be used in a conventional system having inertial devices, such as for example reaction wheels, in combination with chemical thrusters for desaturating the reaction wheels.
Preferably, in order to feed propellant gas to ion thrusters, the gas is stored in pressurized tanks. Nevertheless, a drawback encountered in this field is that of regulating the very low flow rate of the propellant gas feeding the ion thruster from such a pressurized tank. This is particularly difficult given that the pressure within the pressurized tank decreases progressively as the tank is emptied, and that it can be advantageous to regulate this flow rate not to a level that is constant, but rather to a plurality of different levels or to a level that is variable, so as to adapt the performance of the ion thruster to a plurality of different situations. The use of variable flow rate restrictors or valves would have the drawback of increasing the mechanical complexity of the feed device, with that being particularly problematic in a space environment since that environment is particularly hostile for mechanical devices having moving parts.